Transfer the C++03 move-only type emulation into base/move.h and also make ScopedVector move-only.
Also:
* Add a lot of documentation explaining what this macro does.
* Change the implementation of RValue so it cannot be instantiated.
The change to always use RValue& makes for more efficent code in debug builds. Looking at the disassembly for a simple use case (calling a function with one parameter), it removes the creation of one temporary.
BUG=96118
TEST=new unittests. exist code still compiles.
Review URL: https://chromiumcodereview.appspot.com/9207021
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@118388 0039d316-1c4b-4281-b951-d872f2087c98
CrOS-Libchrome-Original-Commit: 59e639aeb0da096e017d0786cfe852319152d74b
diff --git a/base/move.h b/base/move.h
new file mode 100644
index 0000000..f9408f7
--- /dev/null
+++ b/base/move.h
@@ -0,0 +1,211 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_MOVE_H_
+#define BASE_MOVE_H_
+
+// Macro with the boilerplate that makes a type move-only in C++03.
+//
+// USAGE
+//
+// This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create
+// a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be
+// the first line in a class declaration.
+//
+// A class using this macro must call .Pass() (or somehow be an r-value already)
+// before it can be:
+//
+// * Passed as a function argument
+// * Used as the right-hand side of an assignment
+// * Return from a function
+//
+// Each class will still need to define their own "move constructor" and "move
+// operator=" to make this useful. Here's an example of the macro, the move
+// constructor, and the move operator= from the scoped_ptr class:
+//
+// template <typename T>
+// class scoped_ptr {
+// MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
+// public:
+// scoped_ptr(RValue& other) : ptr_(other.release()) { }
+// scoped_ptr& operator=(RValue& other) {
+// swap(other);
+// return *this;
+// }
+// };
+//
+// Note that the constructor must NOT be marked explicit.
+//
+// For consistency, the second parameter to the macro should always be RValue
+// unless you have a strong reason to do otherwise. It is only exposed as a
+// macro parameter so that the move constructor and move operator= don't look
+// like they're using a phantom type.
+//
+//
+// HOW THIS WORKS
+//
+// For a thorough explanation of this technique, see:
+//
+// http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
+//
+// The summary is that we take advantage of 2 properties:
+//
+// 1) non-const references will not bind to r-values.
+// 2) C++ can apply one user-defined conversion when initializing a
+// variable.
+//
+// The first lets us disable the copy constructor and assignment operator
+// by declaring private version of them with a non-const reference parameter.
+//
+// For l-values, direct initialization still fails like in
+// DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment
+// operators are private.
+//
+// For r-values, the situation is different. The copy constructor and
+// assignment operator are not viable due to (1), so we are trying to call
+// a non-existent constructor and non-existing operator= rather than a private
+// one. Since we have not committed an error quite yet, we can provide an
+// alternate conversion sequence and a constructor. We add
+//
+// * a private struct named "RValue"
+// * a user-defined conversion "operator RValue&()"
+// * a "move constructor" and "move operator=" that take the RValue& as
+// their sole parameter.
+//
+// Only r-values will trigger this sequence and execute our "move constructor"
+// or "move operator=." L-values will match the private copy constructor and
+// operator= first giving a "private in this context" error. This combination
+// gives us a move-only type.
+//
+// For signaling a destructive transfer of data from an l-value, we provide a
+// method named Pass() which creates an r-value for the current instance
+// triggering the move constructor or move operator=.
+//
+// Other ways to get r-values is to use the result of an expression like a
+// function call.
+//
+// Here's an example with comments explaining what gets triggered where:
+//
+// class Foo {
+// MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);
+//
+// public:
+// ... API ...
+// Foo(RValue& other); // Move constructor.
+// Foo& operator=(RValue& rhs); // Move operator=
+// };
+//
+// Foo MakeFoo(); // Function that returns a Foo.
+//
+// Foo f;
+// Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context.
+// Foo f_assign;
+// f_assign = f; // ERROR: operator=(Foo&) is private in this context.
+//
+//
+// Foo f(MakeFoo()); // R-value so alternate conversion executed.
+// Foo f_copy(f.Pass()); // R-value so alternate conversion executed.
+// f = f_copy.Pass(); // R-value so alternate conversion executed.
+//
+//
+// IMPLEMENTATION SUBTLETIES WITH RValue
+//
+// The RValue struct has subtle properties:
+//
+// 1) All its methods are declared, but intentionally not defined.
+// 2) It is *never* instantiated.
+// 3) It is a child of the move-only type.
+//
+// (1) is a guard against accidental violation of (2). If an instance of
+// RValue were ever created, either as a temporary, or as a copy to some
+// function parameter or field of a class, the binary will not link.
+//
+// This ensures that RValue can only exist as a temporary which is important
+// to avoid accidental danging references.
+//
+// (3) allows us to get around instantiations because our user-defined
+// conversion can return a downcast of this pointer.
+//
+// operator RValue&() { return *reinterpret_cast<RValue*>(this); }
+//
+// Because RValue does not extend the object size or add any virtual methods,
+// this type-pun is safe.
+//
+// An alternative implementation would be to make RValue into a concrete
+// struct that holds a reference to the type. But in the non-optimized build,
+// this causes unnecessary temporaries to be made bloating the object files.
+// Also, it would then be possible to accidentally persist an RValue instance.
+//
+//
+// COMPARED TO C++11
+//
+// In C++11, you would implement this functionality using an r-value reference
+// and our .Pass() method would be replaced with a call to std::move().
+//
+// This emulation also has a deficiency where it uses up the single
+// user-defined conversion allowed by C++ during initialization. This can
+// cause problems in some API edge cases. For instance, in scoped_ptr, it is
+// impossible to make an function "void Foo(scoped_ptr<Parent> p)" accept a
+// value of type scoped_ptr<Child> even if you add a constructor to
+// scoped_ptr<> that would make it look like it should work. C++11 does not
+// have this deficiency.
+//
+//
+// COMPARED TO Boost.Move
+//
+// Our implementation is based on Boost.Move, but we keep the RValue struct
+// private to the move-only type.
+//
+// In Boost.Move, RValue is the boost::rv<> template. This type can be used
+// when writing APIs like:
+//
+// void MyFunc(boost::rv<Foo>& f)
+//
+// that can take advantage of rv<> to avoid extra copies of a type. However you
+// would still be able to call this version of MyFunc with an l-value:
+//
+// Foo f;
+// MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass().
+//
+// unless someone is very careful to also declare a parallel override like:
+//
+// void MyFunc(const Foo& f)
+//
+// that would catch the l-values first. This was declared unsafe in C++11 and
+// a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot
+// ensure this in C++03.
+//
+// Since we have no need for writing such APIs yet, our implementation keeps
+// RValue private and uses a .Pass() method to do the conversion instead of
+// trying to write a version of "std::move()." Writing an API like std::move()
+// would require the RValue structs to be public.
+//
+//
+// CAVEATS
+//
+// If you include a move-only type as a field inside a class that does not
+// explicitly declare a copy constructor, the containing class's implicit
+// copy constructor will change from Containing(const Containing&) to
+// Containing(Containing&). This can cause some unexpected errors.
+//
+// http://llvm.org/bugs/show_bug.cgi?id=11528
+//
+// The workaround is to explicitly declare your copy constructor.
+//
+#define MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
+ private: \
+ struct rvalue_type : public type { \
+ rvalue_type(); \
+ ~rvalue_type(); \
+ rvalue_type(const rvalue_type&); \
+ void operator=(const rvalue_type&); \
+ }; \
+ type(type&); \
+ void operator=(type&); \
+ public: \
+ operator rvalue_type&() { return *reinterpret_cast<rvalue_type*>(this); } \
+ type Pass() { return type(*reinterpret_cast<rvalue_type*>(this)); } \
+ private:
+
+#endif // BASE_MOVE_H_